1. Field of the Invention
The present invention relates to a process of preparing polyamides having a high melting temperature; more particularly, the present invention relates to a process of preparing dodecane terephthalamide polymers.
2. Description of the Prior Art
Conventional aliphatic polyamide, commonly known as nylons, are semi-crystalline thermoplastic polymers that are noted for their useful properties such as chemical resistance, hardness, high tensile strength, toughness, rigidity and relatively high heat stability. However, conventional polyamides are hygroscopic, that is, they have a tendency to absorb moisture from their environment.
The polyamides derived from terephthalic acid and alkanediamines, which are commonly referred in the art as polyterephthalamides, have been known in the art to exhibit a high melting point and low moisture sensitivity. However, the high melting point of polyterephthalamides, which is higher than 300.degree. C. and practically coincides with the thermal decomposition temperature of the polymers, renders the production of the polymers by the conventional melt polymerization process, which is the most widely used polyamide production process, unpracticable. Various publications recognize this polymerization difficulty. For example, U.S. Pat. No. 2,752,328 to Magat discloses a polyterephthalamide composition with a relatively low polymerization temperature, utilizing diaminoalkanes of 6 to 8 carbon atoms with one or two methyl side chains in place of the traditional straight chain diaminoalkanes. U.S. Pat. No. 3,382,216 to Blaschke et al. discloses a melt-processible, non-crystalline copolyamide of hexamethylene diamine and a mixture of isophthalic acid and terephthalic acid, having a softening point and melting point far below those of polyhexamethylene terephthalamide. U.S. Pat. NO. 3,696,074 to Tsuda et al. discloses terephthalamide copolymer compositions of dodecamethylene diamine, hexamethylene diamine, terephthalic acid and adipic acid, which copolyamides are crystalline and have a range of melting points between 200.degree. C. and 280 .degree. C.
The difficulty in producing polyterephthalamides is further illustrated by U.S. Pat. No. 3,839,296 to Campbell and U.S. Pat. No. 3,917,561 to Chapman et al. Campbell discloses an elaborate melt polymerization procedure for producing dodecane terephthalamide copolymers, which utilizes a carefully monitored sequential polymerization process. Chapman discloses a process for producing dodecane terephthalamide polymers that comprises melt polymerizing cation-exchange treated dodecane terephthalamide salts in the presence of a sterically hindered phenol, benzenephosphinic acid, or acetate in combination with an alkali metal halide. The resulting polymer is said to be melt-stable and melt-spinnable by conventional processes into a useful yarn.
It is clear from the above disclosures that polyterephthalamides that exhibit the desirable properties are not readily produced by the conventional melt polymerization process unless elaborate measures are taken to guard against thermal degradation of the polymers. Therefore, it is desirable to have a practical manufacturing process for polyterephthalamides that can be readily utilized in a manufacturing environment of a commercial scale.
An alternate method of producing polyamide known in the art is continuous polycondensation processes utilizing a continuous reactor or an extruder. The continuous polycondensating reaction involves heating monomeric or prepolymeric starting materials to cause progressive polymerization with loss of low molecular weight volatile material until the resulting polymer achieves desired molecular weight. For example, U.S. Pat. No. 3,040,005 to Bernhardt et al discloses a method of producing nylon 6,6 and copolymers thereof by an extrusion polycondensation process, wherein an inert gas, such as nitrogen, must be supplied to the polycondensating extruder in order to facilitate the proper nylon polymerization. U.S. Pat. No. 4,760,129 to Haering et al. discloses a process of preparing nylon 6,6 in a twin screw extruder by providing serially arranged, alternatingly elevated and reduced zones for pressure and temperature within the extruder. European Patent Application 0 410 649 to Taylor et al discloses a process for producing polyamides of high molecular weight from dicarboxy terminated prepolymers of relatively low molecular weight by further polymerization in a twin screw extruder in the presence of a diamine monomer solution.
U.S. Pat. No. 4,863,991 to Poppe et al. discloses a copolymer composition of hexamethylene terephthalamide that is produced by a twin screw extruder, utilizing high shear mixing and venting capabilities of a twin screw extruder. Although the disclosure indicates that copolymers of hexamethylene terephthalamide can be produced by a twin screw extruder polycondensation process, dodecane terephthalamide polymers have not been produced by such a polycondensation process since the two polyamides do not have the same reaction characteristics. For example, the reaction of dodecane terephthalamide prepolymer formation is slower and the water solubility of dodecane terephthalamide prepolymers is lower than those of hexamethylene terephthalamide prepolymers. Consequently, the reaction of dodecane terephthalamide prepolymer formation requires a longer reaction time and significantly different pressure and temperature conditions.
The Poppe disclosure indicates that the polycondensation of hexamethylene terephthalamide polymers is achieved by a twin screw extruder. The use of a twin screw extruder is dictated by the fact that the viscosity of the prepolymer substrate mixture increases and the efficiency of the polycondensation process decreases as the polycondensation progresses, creating difficulties in the proper heat distribution in the reactor (extruder) and in the removal of the low molecular weight volatile. As a consequence of such processing difficulties, there is a tendency for the polycondensation process to reverse or not to proceed to normal completion, thereby producing an inferior quality polymer. Due to these problems and the limitations of a single screw extruder, such as limited mixing and venting capabilities, successful attempts to use a single screw extruder to polymerize various polyterephthalamides have not been reported. Unlike a twin screw extruder, a single screw extruder does not provide a high mixing shear that can eliminate the heat distribution difficulty in the extruder and does not have auxiliary ports that can be adapted to remove the volatiles produced from the polycondensation reaction. However, single screw extruders are significantly less expensive and more widely available than twin screw extruders.
The present invention provides a practical process for preparing dodecane terephthalamide polymers that can be achieved in a widely available and less expensive equipment. In addition, the present invention provides a process for preparing dodecane terephthalamide polymers that can be utilized in a manufacturing environment of a commercial scale. The present invention also provides a process for preparing dodecane terephthalamide polymers that does not require a costly vacuum or inert gas purging procedure.